Synthesis and X-ray structural characterization of sterically hindered bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complexes

Article abstract of DOI:10.1016/j.poly.2012.03.006

Bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complexes were prepared in high yields from a sterically hindered 3,5-di-tert-butylsalicylaldehyde and a variety of amines using three synthetic approaches. They were all thoroughly characterized by various spectroscopic methods (UV-Vis, IR, EPR and EI-MS). Crystal structures of seven complexes determined by X-ray crystallography show the Cu ion in a N₂O₂ coordination environment and a tetrahedrally distorted square-planar geometry. The UV-Vis and EPR results indicated that the complexes have the same geometry also in solution. In two solid state structures of the complexes the ligands have obtained an unusual cis-orientation whereas the rest of the complexes are trans-isomers. Computational studies carried out by the density functional theory method verified that in the case of complex with N-alkyl fragment the preferred isomer is trans, while the opposite behavior is observed for the complex with N-phenyl substituent.

Full text of DOI:10.1016/j.poly.2012.03.006

Polyhedron 38 (2012) 205–212
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and X-ray structural characterization of sterically hindered
bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complexes
Jahir Uddin Ahmad, Minna T. Räisänen, Martin Nieger, Markku R. Sundberg, Pawel J. Figiel,
⇑
Markku Leskelä, Timo Repo
Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, University of Helsinki, FI-00014 Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complexes were prepared in high yields from a sterically
hindered 3,5-di-tert-butylsalicylaldehyde and a variety of amines using three synthetic approaches. They
were all thoroughly characterized by various spectroscopic methods (UV–Vis, IR, EPR and EI-MS). Crystal
structures of seven complexes determined by X-ray crystallography show the Cu ion in a N2O2 coordina-
tion environment and a tetrahedrally distorted square–planar geometry. The UV–Vis and EPR results
indicated that the complexes have the same geometry also in solution. In two solid state structures of
the complexes the ligands have obtained an unusual cis-orientation whereas the rest of the complexes
are trans-isomers. Computational studies carried out by the density functional theory method verified
that in the case of complex with N-alkyl fragment the preferred isomer is trans, while the opposite behav-
ior is observed for the complex with N-phenyl substituent.
Received 10 January 2012
Accepted 4 March 2012
Available online 10 March 2012
Keywords:
Schiff base
Cu(II) complex
Salicylaldimine
Crystal structure
cis–trans Isomer
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Cu(II) Schiff base complexes [6a,c,7], we report herein the synthe-
sis and characterization of sterically hindered bis(3,5-di-tert-
Schiff base metal complexes are constantly used in numerous
fields of applications varying from catalysis to pharmaceuticals.
For instance, transition metal Schiff base complexes catalyze cyclo-
propanation, aziridination, oxidation, epoxidation, polymerization
and hetero-Diels–Alder reactions. Cu complexes of Schiff base
derivatives are also an important class of bioactive compounds as
they exhibit good antibacterial and antimalarial activities [1].
Salicylaldimine ligands have been reported to enhance metal
ion chelation and provide additional stability to the metal center
[2]. Introduction of bulky tert-butyl substituents to the ligands
may cause significant changes in the chemical properties of the
resulting complexes [3]. Due to their many potential applications,
crystallographic data of Cu(II) complexes with salicylaldimine li-
gands having two bulky substituents are of importance. So far,
there are several structural studies of Cu(II) complexes with differ-
ently substituted bi-, tri- and tetra-dentate salicylaldimine ligands
[4]. However, structures of complexes having two bidentate li-
gands specifically with two tert-butyl substituents in the salicyl-
aldimine moieties are still rare [5]. Recently, we have studied
such Cu(II) complexes as catalysts in the aerobic oxidation of ben-
zylic alcohols and the corresponding ligand structures in solution
and solid state [6]. In continuation to our ongoing research with
butylsalicylaldiminato)Cu(II) complexes. Seven of these complexes
were structurally characterized by X-ray crystallography.
2. Experimental
2.1. General
All chemicals were obtained from commercial suppliers and
used without further purification. Melting points were determined
in an electrothermal melting point apparatus and EI-mass spectra
were run with a JEOL JMS–SX 102 mass spectrometer (ionization
voltage 70 eV) from solid samples. IR spectra were collected with
Perkin Elmer Spectrum GX spectrometer and elemental analyses
were made by an EA 1110 CHNS-OCE instrument. UV–Vis spectra
were recorded with a Hewlett Packard 8453 spectrophotometer
from toluene solution.
The yields of 2, 3 and 5–9 prepared from bis(3,5-di-tert-buty-
lsalicylaldehydato)Cu(II) (1) [8] intermediate were not optimized.
Their purities were verified by elemental analysis and melting
point measurements. The complex formations were also confirmed
by IR spectroscopy. The C@O band (1615–1640 cmꢀ1) observed in
the spectra of the complex precursor 1 disappeared upon the
bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complex formation
and the characteristic C@N band at 1614–1629 cmꢀ1 appeared.
Detailed synthesis and analysis of the ligands and complexes 2,
5 and 8 can be found in elsewhere [6]. The yields of the complexes
⇑
Corresponding author. Tel.: +358 9191 50194; fax: +358 9191 50198.
E-mail address: timo.repo@helsinki.fi (T. Repo).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.03.006

206
J.U. Ahmad et al. / Polyhedron 38 (2012) 205–212
varied (2: 83% or 80%. 5: 86%, 91%, 90% or 96%. 8: 85%, 85%, or 93%)
depending on the used synthetic procedure.
2.2.4. Bis[(N-benzyl-3,5-di-tert-butylsalicylaldiminato)]Cu(II) (7)
Brown plate-like crystals of 7 suitable for X-ray structure deter-
mination were formed in a reaction mixture after a few days stor-
age at room temperature. Single crystals with toluene solvent
molecule were grown in a refrigerator by a slow evaporation of a
saturated toluene solution. (mp 128–130 °C). Yield 91%, 87% or
92% depending on the synthetic procedure. Anal. Calc. for
2.2. Synthesis of bis(salicylaldiminato)Cu(II) complexes
A methanol (60 mL) solution of Cu(CH3COO)2 (1.26 mmol) was
slowly added in a solution of ligand (2.5 mmol in 15 mL MeOH)
and stirred for 1 h at room temperature. The brown solid was sep-
arated with suction filtration and recrystallized from a mixture of
MeOH and CH2Cl2. This preparation method was applicable for
synthesis of 2–8.
Alternatively, complex precursor 1 (265 mg, 0.5 mmol) was sus-
pended in EtOH (10 mL) and the corresponding amine (1 mmol)
was added. The resulting mixture was refluxed for 6 h and the solid
product formed was separated with suction filtration and recrys-
tallized from a mixture of MeOH and CH2Cl2. Complexes 2, 3 and
5–9 could be synthesized with this procedure.
In one-pot synthesis of the complexes, a methanol (30 mL) solu-
tion of Cu(CH3COO)2 (181.1 mg, 1 mmol) was added dropwise to a
stirred MeOH (5 mL) solution of 3,5-di-tert-butylsalicylaldehyde
(468 mg, 2 mmol) and Et3N (2 equiv.). The greenish-brown solid
was formed at room temperature. The corresponding amine
(2 mmol) was then added and the resulting mixture was heated
to reflux for 6 h. During the course of the reaction, the solid was
dissolved and the color of the solution changed from green to
brown, which indicates the in situ formation of the copper Schiff
base complex. The product formed was separated with suction fil-
tration and recrystallized from a mixture of MeOH and CH2Cl2. This
synthetic procedure was applicable for the preparation of 2, 3 and
5–9.
C
44H56CuN2O2 (708.5 g/mol): C, 74.59; H, 7.97; N, 3.95. Found: C,
74.18; H, 8.15; N, 3.80%. IR (cmꢀ1): 2952–2867 (
C–H from tert-bu-
tyl groups), 1617 ( C@N), 1433 ( C–O).UV–Vis kmax (nm): 284, 328,
m
m
m
385, 486, 657. MS (EI): m/z = 707–710 with appropriate isotopic ra-
tio for [C44H56CuN2O2]+.
2.2.5. Bis[(N-n-hexyl-3,5-di-tert-butylsalicylaldiminato)]Cu(II) (9)
Synthesis and detailed analyses of 9 can be found in the litera-
ture [10]. Brown crystals of 9 suitable for X-ray structure determi-
nation were obtained by a slow layer diffusion of DMSO into a
CH2Cl2 solution of 9 at room temperature (mp 155–158 °C, litera-
ture mp 193 °C). Yield: 86% or 95% depending on the synthetic pro-
cedure. Anal. Calc. for C42H68CuN2O2 (696.5 g/mol): C, 72.42; H,
9.84; N, 4.02. Found: C, 72.48; H, 9.63; N, 4.03%. IR (cmꢀ1): 2956–
2854 (mC–H from tert-butyl groups), 1619 (mC@N), 1433 (mC–O). UV–
Vis kmax (nm): 284, 327, 380, 486, 657. MS (EI): m/z = 696–699 with
appropriate isotopic ratio for [C42H68CuN2O2]+.
2.3. EPR spectroscopy
X-band EPR spectra of Cu(II) complexes 2, 5 and 6 were mea-
sured using 10 lM toluene solutions of the complexes. Experi-
ments were carried out on a Bruker ESP 300E spectrophotometer
equipped with the Bruker NMR gaussmeter ER035M and the Hew-
lett–Packard microwave frequency counter with 100 kHz field
modulation and microwave power of 20 mW at room temperature
(quartz capillary tubes) and at 77 K (quartz tube, internal diameter
ca. 1.7 mm, liquid N2). Simulations of the EPR spectra were per-
formed using Bruker WinEPR Simfonia software in order to calcu-
late the A and g values.
2.2.1. Bis[(N-p-methyphenyl-3,5-di-tert-butylsalicylaldiminato)]Cu(II)
(3)
Synthesis and detailed analyses of 3 can be found in the literature
[9]. Brown crystals of 3 suitable for X-ray structure determination
were grown in a refrigerator by a slow evaporation of a saturated
toluene solution (mp 269–273 °C, literature mp > 260 °C). Yield
86%, 82% or 90% depending on the synthetic procedure. Anal. Calc.
for C44H56CuN2O2 (708.5 g/mol): C, 74.59; H, 7.97; N, 3.95. Found:
C, 75.34; H, 7.86; N, 4.13%. IR (cmꢀ1): 2956–2867 (
tyl groups), 1615 ( C@N), 1427 ( C–O). UV–Vis kmax (nm): 296, 368,
mC–H from tert-bu-
2.4. X-ray crystallography studies
m
m
Single-crystal X-ray diffraction studies of3–4, 6–7, 7T (molecule
crystallized with toluene solvent) and 9 were carried out on a Bru-
421, 597, 628. MS (EI): m/z = 707–711 with appropriate isotopic ra-
tio for [C44H56CuN2O2]+.
ker–Nonius Kappa-CCD diffractometer using Mo K
a radiation
(k = 0.71073 Å). Direct methods (SHELXS-97) were used for structure
solution and refinement (SHELXL-97), full-matrix least-squares on F2
[11]. Semi-empirical absorption corrections were applied, and H
atoms were localized by difference electron density determination
and refined using a riding model. Details of the data collection and
structure refinement are given in Table 1 and in footnotes.1,2 The
data obtained for 4 are not of very high quality but allows the discus-
2.2.2. Bis[(N-p-nitrophenyl-3,5-di-tert-butylsalicylaldiminato)]Cu(II)
(4)
Brown crystals of 4 suitable for X-ray structure determination
were grown by a slow evaporation of a saturated CH2Cl2 solution
in DMSO (mp 260–262 °C). Yield: 78%. Anal. Calc. for C42H50CuN4O6
(770.4 g/mol): C, 65.48; H, 6.54; N, 7.27. Found: C, 65.57; H, 6.69; N
6.91%. IR (cmꢀ1): 2952–2868 (mC–H from tert-butyl groups), 1521
(mC@N), 1421 (mC–O). UV–Vis kmax (nm): 287, 317, 437, 583, 628.
1
7: C44H56CuN2O2, M = 708.45, data collected at 123(2) K, crystal size
MS (EI): m/z = 770–773 with appropriate isotopic ratio for
[C42H50CuN4O6]+.
0.50 ꢁ 0.35 ꢁ 0.25, monoclinic, space group P21 /n (No. 14): a = 20.605(2) Å,
b = 23.360(2) Å, c = 26.541(2) Å, b = 95.56(1)°, V = 12714.9(19) Å3, Z = 12,
qCalc = 1.110
mgmꢀ3
,
F (000) = 4548,
l
= 0.550 mmꢀ1
,
74066 reflections (2hmax = 50°), 22177
(I)) = 0.070, wR2 (all
2.2.3. Bis[(N-cyclohexyl-3,5-di-tert-butylsalicylaldiminato)]Cu(II) (6)
Synthesis and detailed analyses of 6 can be found in the litera-
ture [10]. Brown needles of 6 suitable for X-ray structure determi-
nation were grown at room temperature by slow layer diffusion of
MeOH into a CH2Cl2 solution of 6. (mp 250–252 °C; literature mp
151 °C). Yield: 91%, 89% or 93% depending on the synthetic proce-
dure. Anal. Calc. for C42H64CuN2O2 (692.5 g/mol): C, 72.84; H, 9.32;
N, 4.05. Found: C, 71.60; H, 8.54; N, 3.95%. IR (cmꢀ1): 2931–2855
unique [Rint = 0.098], 1324 parameters, 0 restraints, R1 (I > 2
r
data) = 0.143, GOF = 1.01, largest difference in peak and hole 0.777/ꢀ0.657 eÅꢀ3. Use
of SIMU and DELU restraints did not improve the quality of the crystal structure (they
had an opposite effect). There is a slight disorder in some tert-butyl and phenyl groups
which could not be resolved reasonably.
2
4: C42H50CuN4O6, M = 770.40, data collected at 123(2) K, crystal size
0.08 ꢁ 0.30 ꢁ 0.40, orthorhombic, space group Pccn (No. 56): a = 24.641(29) Å,
b = 30.417(3) Å, c = 12.039(1) Å, V = 9023.3(14) Å3, Z = 8,
q , F
Calc = 1.134 mgmꢀ3
43523 reflections (2hmax = 50°), 7892 unique
(I)) = 0.150, wR2 (all data) = 0.394,
(000) = 3256,
[Rint = 0.122], 478 parameters, 0 restraints, R1 (I > 2
l
= 0.529 mmꢀ1
,
r
(mC–H from tert-butyl groups), 1615 (mC@N), 1436 (mC–O). UV–Vis kmax
GOF = 1.18, largest difference in peak and hole 1.657/ꢀ0.693 eÅꢀ3. Forty electrons
from diffuse solvent found in voids. Use of SQUEEZE did not improve the structure
significantly.
(nm): 284, 328, 379, 487, 657. MS (EI): m/z = 693–696 with appro-
priate isotopic ratio for [C42H64CuN2O2]+.

J.U. Ahmad et al. / Polyhedron 38 (2012) 205–212
207
Table 1
Crystallographic data for 3, 6, 7T and 9.
3
6
7T
9
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimension
a (Å)
C
44H56Cu N2O2 – 1.5 toluene
C42H64CuN2O2
692.49
173(2)
orthorhombic
Pbcn (No. 60)
C44H56CuN2O2 – toluene
800.58
123(2)
monoclinic
P21/n (No. 14)
C42H68CuN2O2
696.52
123(2)
monoclinic
P21 (No. 4)
846.65
123(2)
triclinic
ꢀ
P1 (No. 2)
10.906(1)
15.114(2)
15.583(3)
112.20(1)
90.41(1)
90.54(1)
2377.9(6)
2
29.629(6)
12.913(3)
21.370(4)
90
90
90
8176(3)
8
1.125
0.568
3000
18.488(2)
9.367(1)
26.192(4)
90
99.23(1)
90
4477.1(10)
4
1.188
9.356(1)
10.923(1)
20.045(2)
90
94.68(1)
90
2041.7(4)
2
1.133
0.569
758
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
DCalc (gcmꢀ3
)
1.182
0.501
908
Absorption coefficient (mmꢀ1
F(000)
)
0.528
1716
Crystal dimensions (mm)
2hmax (°)
0.45 ꢁ 0.25 ꢁ 0.10
0.35 ꢁ 0.10 ꢁ 0.10
0.55 ꢁ 0.30 ꢁ 0.15
0.50 ꢁ 0.25 ꢁ 0.15
55
50
50
55
Number of collected data
Number of unique data
Rint.
53788
10754
0.032
44021
7314
0.076
55400
7832
0.047
22118
9303
0.045
Number of parameters/restraints
537/36
0.032
0.084
1.04
0.562/ꢀ0.451
421/0
0.042
0.102
1.02
484/149
0.057
0.140
1.05
1.254/ꢀ1.012
424/26
0.058
0.130
1.03
1.239/ꢀ0.607
Ra[I > 2
r(I)]
b
wR2
Goodness-of-fit (GOF) on F2
Largest difference peak and hole (eAꢀ3
)
0.309/ꢀ0.308
a
R =
wR2 = [
R
||Fo|-|Fc|| /
R
R |Fo|.
b
[w(Fo2-Fc2)2] /
R
[w(Fo2)2]]1/2
.
Scheme 1. Synthesis of bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complexes (2–9).
sion about the isomeric form of the complex. In 4 there are 40 elec-
trons localized in voids (diffuse electron density of solvent DMSO) by
using SQUEEZE [12]. For 9 the absolute structure was determined by
refinement of Flack’s x-parameter (x = ꢀ0.005(13)).
Table 2
EPR parameters for 2, 5 and 6 in toluene.
Aiso [G]a
g||
A|| [G]b
g\
g||/A|| [cm]
a
b
b
Complex
giso
2
5
6
2.116
2.124
2.117
59
71
67
2.235
2.238
2.231
167
170
170
2.042
2.049
2.046
133.83
131.65
131.24
2.5. Computational studies
a
Room temperature.
77 K.
b
Optimizations of molecular structures were carried out at the
B3LYP/6-311G⁄ level of theory. Symmetry constraints were not ap-
plied and imaginary frequencies were not found at the computed
IR spectra. The program suite employed was GAUSSIAN09 [13].
lsalicylaldiminato)Cu(II) complexes (2–9) can be synthesized in
good yields using three different approaches (Scheme 1). In the
method which is the most commonly used synthetic approach
for these types of complexes, the firstly prepared 3,5-di-tert-butyl-
salicylaldimine is reacted with Cu(II) acetate in a 2:1 M ratio at
room temperature to obtain complexes 2–8. In another method
the bis(3,5-di-tert-butylsalicylaldehydato)Cu(II) complex precur-
sor (1) is prepared by mixing the aldehyde and Cu(II) acetate
(2:1) in the presence of Et3N at room temperature [8]. Then 1 is
3. Results and discussion
3.1. Synthesis of ligands and complexes
Ligands used in this study were all prepared following the liter-
ature procedures [6,14]. Sterically hindered bis(3,5-di-tert-buty-

208
J.U. Ahmad et al. / Polyhedron 38 (2012) 205–212
Fig. 1. Molecular structures of complexes (a) 2 [6a] and (b) 3 with the atom numbering schemes. Displacement parameters are drawn at 50% probability level. Toluene
solvent molecules of 3 and hydrogen atoms are omitted for clarity.
Fig. 2. Molecular structures of (a) 4 and (b) 5 [6c] with the atom numbering schemes. Displacement parameters are drawn at 50% probability level. Hydrogen atoms and the
disorder in one of the tert-butyl groups of 5 are omitted for clarity.
converted to the bis(3,5-di-tert-butylsalicylaldiminato) complexes
(2, 3 and 5–9) by adding amine in refluxing condition. Alterna-
tively, the complexes (2, 3 and 5–9) can be obtained using the
developed facile one-pot synthesis. First aldehyde, Cu(II) acetate
and Et3N are reacted in 1:2:2 M ratio at room temperature to gen-
erate in situ the precursor 1. Then 1 is converted into the aimed
complex simply by refluxing it with the appropriate amine (ami-
ne:1 M ratio of 1:2).
It is noteworthy that with alkyl and phenylalkyl amines the
transformation of 1 into the corresponding imine complex (2–9)
is more facile than with phenyl amine, and that the transformation
was not successful with aryl amine having an electron withdraw-
ing NO2 group at para position. It is likely that the electron with-
drawing group, which reduces the basicity of amine nitrogen,
causes the slower reaction rate or even prevents the imine forma-
tion altogether. Moreover, aryl amine with an electron releasing
methyl group has better reactivity than phenyl amine.
The infrared spectra of the free ligands show a stretching vibra-
tion band between 2949 and 2966 cmꢀ1 indicating the presence of
phenolic hydroxyl group [6b]. The absence of this band in the IR
spectra of the Cu(II) complexes indicates deprotonation of the li-
gand and formation of Cu–O bond [10]. The C@N stretching vibra-
tion observed for the ligands between 1570 and 1635 cmꢀ1 is
shifted in the complexes to lower frequencies (1520 and
1629 cmꢀ1) [6b]. This is indicative of Cu–N coordination bond for-
mation [15]. In the complex spectra bands are observed also in the
540–550 cmꢀ1 and 500–430 cmꢀ1 regions due to Cu–O and Cu–N
stretching vibration modes, respectively [10].
Three complexes with different electronic and steric effects
were chosen for EPR studies to compare their solid state and solu-
tion structures. The X-band EPR spectra of 2, 5 and 6 in toluene
show a typical pattern for monomeric distorted square–planar
Cu(II) complexes (Table 2). The EPR spectra in toluene at room
temperature exhibit four-line isotropic signals with giso 2.116–
2.124 and Aiso 59–71 G, being in good agreement with other re-
ports on Cu(II) complexes with salicylaldimine ligands [10]. The
frozen toluene solutions of Cu(II) salicylaldimine complexes dis-
play four lines corresponding to Cu hfs, (gzz > gxx = gyy, ACu = 167–
170 G), spectra parameters were obtained from simulations and
they are consistent with spectra observed for well-known distorted
tetrahedral Cu(II) complexes [16]. The degree of distortion in com-
plexes 2, 5 and 6 is relatively small as g||/A|| values are 131.24–
133.83 cm [17a]. In 2 bulky phenyl substituents create steric ef-
fects and the structure is most distorted. The results suggest that
3.2. Spectroscopic characterization of complexes
The electronic spectra of the complexes in toluene show bands
in the 284–370 nm and 379–437 nm regions assigned to the
p
?
p⁄ and n ? p⁄ electron transitions of C@C (aromatic) and
C@N (imine) bonds, respectively [9,10,15]. Low-intensity bands
appearing in the 486–597 nm and 628–657 nm regions are related
to d–d transitions of the metal center and are characteristic for dis-
torted square-planar Cu(II) complexes [9,10].

J.U. Ahmad et al. / Polyhedron 38 (2012) 205–212
209
Fig. 3. Molecular structures of (a) 6 and (b) 7 with the atom numbering schemes. Displacement parameters are drawn at 50% probability level and H atoms are omitted for
clarity. For 7 only one crystallographically independent molecule is shown.
complexes 2, 5 and 6 have similar coordination spheres both in
solution and in solid state (see X-ray diffraction studies below).
3.3. Crystal structures
Crystal structures of complexes 3, 6, 7 and 9 were determined
by X-ray crystallography. For complex 7 two different structures,
7
1 and 7T (with toluene solvent molecule in the asymmetric unit),
were obtained. The data obtained for 4 are not of very high quality2
but allow the discussion about the isomeric form of the complex.
Despite of all efforts, single crystals suitable for X-ray structure
determination were not obtained for 8. The structures of 2, 5 and
the ligand precursors used in this work have been shown in our
other papers [6] but for comparison they are shortly discussed also
herein.
3.3.1. Crystal structures of bis(salicylaldiminato)Cu(II) complexes
Bis(salicylaldiminato)Cu(II) complexes 2, 3, 5–7, 7T and 9 have a
distorted square-planar geometry where the coordination sphere
around the Cu(II) consists of the N2O2 donor atoms of the bidentate
ligands (Figs. 1–4). In the structure of 3, there is an additional
interaction between the Cu(II) and H atom of toluene solvent mol-
ecule. The Cuꢂ ꢂ ꢂH distance (3.00 Å) is shorter than the sum of the
corresponding van der Waals radii (rCuꢂ ꢂ ꢂH = 3.117 Å), indicating a
non-bonded interaction [17]. In two complexes, 2 [6a] and 4, the
ligands are in an unusual cis-configuration with respect to each
other whereas rest of the complexes (3, 5 [6c]-7, 7T, 9) are typical
trans-isomers. Complexes 2 and 4 have Ph and p-NO2Ph substitu-
ents, respectively, at the imine nitrogens. It is likely that the driv-
Fig. 4. Molecular structure of 9 with the atom numbering scheme. Displacement
parameters are drawn at 50% probability level. Hydrogen atoms are omitted for
clarity.
tert-butylsalicylaldiminato)Cu(II), show the Cu centers in
a
distorted square-planar geometry with typical Cu–N (1.959(5)–
1.994(4) Å) and Cu–O (1.875(4)–1.918(4) Å) bond lengths. All these
complexes have the ligands in a trans-configuration [4a,5,19].
The closest intermolecular Cuꢂ ꢂ ꢂCu distances (7.00–7.20 Å) be-
tween the crystallographically independent molecules of 7 are
considerably longer than the Cuꢂ ꢂ ꢂCu van der Waals distance
(2.80 Å) and that of in metallic Cu (2.56 Å) [17,20]. This indicates
that there is no interaction between the Cu centers. Molecular
structure of 7T does not differ significantly from that of 7, except
in 7T one of the tert-butyl groups is disordered and there is one tol-
uene solvent molecule in the asymmetric unit.
ing force for the cis arrangement is
p–p stacking of the Ph
substituents which is observed in both structures.3
However, it is not clear why 3, which has p-MePh substituted at
imine nitrogens, then crystallized as a trans-isomer. Presumably,
interaction between the Cu(II) and H atom of solvent molecule in
3 could affect the geometric arrangement of the corresponding
ligand and thus the ligands adopt a trans-configuration. It is note-
worthy that crystals of 3 suitable for X-ray structural characteriza-
tion could not be obtained from other solvents besides toluene.
Accordingly, the solvent could have an important role during the
crystallization and to the formation of a preferred isomer [18].
Crystal structures of compounds similar to 2–9, namely bis(N-(4-
dimethylaminophenyl)-3,5-di-tert-butylsalicylaldiminato)Cu(II),
bis(N-(4-diethylamino-2-methylphenyl)-3,5-di-tert-butylsalicylal
diminato)Cu(II), bis((R)-N-(1-(cylcohexyl)ethyl)-3,5-di-tert-buty-
lsalicylaldiminato)Cu(II) and bis(N-(2,3,4-trifluorophenyl)-3,5-di-
The Cu(II) complexes 2, 3, 5–7, 7T and 9 show a different degree
of tetrahedral distortion which can be best evaluated by comparing
their N80–O1–N8–O10 (in trans-isomers) and O10–O1–N8–N80 (in
cis-isomers) dihedral angles (
can also be assessed by calculating
D
h, Table 3). The distortion level
P
D
CP = (observed angle ꢀ the-
oretical angle)2, where the angle is O1–Cu–N8, O10–Cu–N80, O1–
Cu–N80, O10–Cu–N8, O1–Cu–O10, and N1–Cu–N80 (Table 3). When
the structures without any solvent of crystallization (2, 5–7, 9)
are compared, the smallest
Dh and DCP values are in the structure
of 5 and the highest in 2. It seems that in these structures the dis-
tortion correlates with sterical effects caused by the N-substituted
groups. The distortion is increasing with the increasing bulkiness
of the N-substituted group as follows: 2 > 6, 7, 9 > 5. However, also
3
In complex 2, the distances between the centers of two N–Ph planes are 3.41 and
3.25 Å. Similarly, they are 3.36 and 3.23 Å in complex 4.

210
J.U. Ahmad et al. / Polyhedron 38 (2012) 205–212
Table 3
Selected geometrical parameters (in Å and °) of metallocyclic part for 2–3, 5–7, 7T and 9. Structures of 2 and 5 have been previously published [6] but they are included here for
comparison reasons.
2
3
5
6
7a
7T
9
Cu–O1
1.9011(12) 1.8923(10) 1.918(2)
1.9123(10) 1.8876(10) 1.907(2)
1.9838(13) 1.9544(11) 1.963(3)
1.9772(13) 1.9576(11) 1.966(3)
1.8976(17) 1.917(3)
1.9106(17) 1.911(3)
1.910(3)
1.915(3)
1.956(3)
1.954(3)
93.32(13)
91.27(14)
91.66(13)
93.32(14)
1.914(3)
1.922(3)
1.951(4)
1.944(4)
90.65(15)
93.27(15)
93.60(15)
92.69(15)
1.905(2)
1.904(2)
1.952(3)
1.945(3)
92.16(10)
92.48(11)
91.22(11)
91.86(10)
1.902(2)
1.900(3)
1.957(3)
1.968(3)
93.52(12)
92.97(11)
92.19(11)
91.40(12)
Cu–O10
Cu–N8
Cu–N80
1.981(2)
1.981(2)
92.97(8)
92.00(8)
91.09(8)
92.96(8)
1.952(3)
1.953(4)
93.13(13)
91.38(14)
91.97(14)
92.99(14)
O1–Cu–N8
O10–Cu–N80
O1–Cu–N80
O10–Cu–N8
O1–Cu–O10
93.24(5)
92.31(5)
150.45(5)
152.82(5)
87.90(5)
99.69(5)
1726
93.65(5)
93.75(5)
94.96(4)
93.54(5)
147.43(5)
151.38(5)
1944
92.79(11)
91.96(11)
91.49(11)
90.99(11)
156.83(10) 155.68(8)
161.91(11) 158.44(9)
156.49(12) 155.38(12) 155.33(12) 156.33(10) 153.92(12)
156.57(14) 157.34(14) 155.88(14) 161.10(12) 157.53(12)
N1–Cu–N80
b
D
D
CP
879
28
1079
31
1126
32
1146
32
1222
32
933
29
1213
32
h: N80–O1–N8–O10 (trans) O10–O1–N8–
N80 (cis)
37
40
a
Three crystallographically independent molecules.
D
P
b
CP = (observed angle ꢀ theoretical angle)2, where the angle is O1–Cu–N8, O10–Cu–N80, O1–Cu–N80, O10–Cu–N8, O1–Cu–O10, and N1–Cu–N80.
Table 4
Comparison of selected bond distances (Å) and angles (°) in Cu(II) complexes (2, 3, 5–7, 7T and 9) The corresponding values found in the structures of free ligands are given in the
parenthesis when available. Structures of the ligands and the complexes 2 and 5 can be found in Ref. [6].
2
3a
5b
6
7c
7T
9
O1–C1
1.307(2)
1.3110(18)
[1.3575(16)]
1.3049(16)
1.3022(16)
[1.352(3)]
[1.355(3)]
1.325(4)
1.326(4)
[1.362(2)]
[1.365(2)]
[1.363(2)]
1.296(4)
1.317(3)
1.312(3)
[1.365(5)]
1.311(5)
1.320(5)
[1.358(3)]
1.297(5)
1.319(5)
[1.358(3)]
1.311(5)
1.308(5)
[1.358(3)]
1.308(4)
1.308(4)
[1.358(3)]
1.316(4)
1.305(4)
N8–C7
1.318(2)
1.3084(19)
[1.2885(18)]
1.2939(18)
1.2944(18)
[1.287(3)]
[1.288(3)]
1.296(3)
1.292(3)
[1.277(5)]
1.280(5)
1.280(5)
[1.282(3)]
1.288(5)
1.275(5)
[1.282(3)]
1.285(6)
1.288(6)
[1.282(3)]
1.279(4)
1.281(4)
[1.282(3)]
1.293(5)
1.279(5)
1.288(4)
[1.283(3)]
[1.286(3)]
[1.285(3)]
1.499(4)
N8–C9
1.4357(19)
1.4404(18)
[1.4234(17)]
1.4228(17)
1.4241(17)
[1.421(3)]
[1.423(3)]
1.491(3)
1.494(3)
[1.464(5)]
1.467(5)
1.471(5)
[1.461(3)]
1.487(5)
1.470(5)
[1.461(3)]
1.477(5)
1.473(6)
[1.461(3)]
1.460(4)
1.461(4)
[1.461(3)]
1.469(5)
1.468(4)
1.494(4)
[1.477(3)]
[1.474(3)]
[1.475(3)]
2.810
2.785
[2.572]
[2.579]
[2.550]
120.1(3)
120.4(3)
O1. . .N8
C7–N8–C9
C6–C7–N8
O1–C1–C6
2.824
2.806
[2.566]
2.806
2.807
[2.585]
[2.563]
2.814
2.800
[2.620]
2.810
2.765
[2.579]
2.812
2.766
[2.579]
2.749
2.811
[2.579]
2.778
2.780
[2.579]
2.811
2.805
116.81(13)
117.64(13)
[123.25(12)]
118.78(11)
117.70(11)
[120.1(2)]
[121.5(2)]
118.4(2)
117.7(2)
[118.5(4)]
114.9(3)
117.0(4)
[118.7(2)]
116.2(4)
117.9(3)
[118.7(2)]
117.3(4)
114.8(4)
[118.7(2)]
116.6(3)
116.8(3)
[118.7(2)]
118.3(3)
118.2(3)
[119.10(18)]
[119.24(18)]
[120.59(19)]
127.0(3)
127.93(14)
127.92(14)
[121.14(12)]
127.36(12)
127.81(12)
[122.2(2)]
[121.3(2)]
128.4(2)
127.4(2)
[123.3(4)]
128.7(4)
127.9(4)
[122.5(2)]
128.5(4)
127.0(4)
[122.5(2)]
126.1(4)
128.7(5)
[122.5(2)]
128.0(3)
127.9(3)
[122.5(2)]
127.8(4)
127.9(3)
125.9(3)
[122.3(2)]
[122.57(19)]
[122.27(19)]
121.1(3)
122.85(14)
122.27(12)
[120.14(11)]
122.59(12)
122.47(12)
[120.0(2)]
[120.3(2)]
122.1(2)
122.4(2)
[119.9(3)]
121.5(4)
122.2(4)
[119.9(2)]
122.2(4)
122.2(4)
[119.9(2)]
122.2(4)
121.6(4)
[119.9(2)]
122.1(3)
121.9(3)
[119.9(2)]
121.5(3)
122.1(3)
121.6(3)
[119.49(18)]
[119.65(18)]
[119.47(18)]
a
b
c
Two crystallographically independent molecules in the asymmetric unit of the ligand structure.
Three crystallographically independent molecules in the asymmetric unit of the ligand structure.
Three crystallographically independent molecules in the asymmetric unit of the complex.

J.U. Ahmad et al. / Polyhedron 38 (2012) 205–212
211
Table 5
creases basicity of amine nitrogen which can further retard the
reaction rate or even prevent the imine formation.
Comparison of the total energies for optimized structures of the selected complexes.
All bis(3,5-di-tert-butylsalicylaldiminato)Cu(II) complexes were
characterized by spectroscopic methods and their crystal struc-
tures, except that of 8, were determined. The structural studies
show the Cu ions in a N2O2 coordination environment with a dis-
torted square–planar geometry. In two complexes the ligands have
adopted a cis-orientation whereas the rest of the complexes are
trans-isomers. Computational studies revealed that in the case of
complex with N-alkyl substituent the preferred isomer is trans,
while the opposite behavior is observed for the N-phenyl contain-
ing complex. The X-band EPR spectra of the complexes in toluene
show a typical pattern for monomeric distorted square–planar
Cu(II) complexes. Thus, it can be concluded that the complexes
have a similar coordination sphere both in solution and in solid
state.
Complex
Energy in Ha
Ediff in kcal/mol
trans-Isomer of 2
cis-Isomer of 2
trans-Isomer of 5
cis-Isomer of 5
ꢀ3532.71318197
ꢀ3532.71382517
ꢀ3306.45065811
ꢀ3306.44270085
0
0.40
4.99
0
the electronic effects into the geometry must be taken into ac-
count, especially in the structures of 3 and 7T where there are
additional Cuꢂ ꢂ ꢂH interactions between the Cu ion and toluene sol-
vent molecules.
As expected, in all structures the Cu–N coordination bonds
(1.944(4)–1.9838(13) Å) are significantly longer than the covalent
Cu–O bonds (1.8876(10)–1.922(3) Å). They are all within the ex-
pected range for Cu(II) Schiff base complexes [8,21]. The C@N bond
lengths (1.275(5)–1.318(2) Å) indicate that the double bond char-
acter is maintained upon the complex formation as the Csp2@N is
1.279(8) Å [22]. The bond distances within the metallocyclic part
Acknowledgments
We gratefully acknowledge financial support from the Graduate
School of Inorganic Materials Chemistry (JUA). We also thank Mrs.
Jadwiga Ke˛dzierska from the University of Wrocław (Poland) for
the assistance in EPR experiments. M.N. thanks the Academy of
Finland (Proj. No.: 128800). Generous computing time given by
CSC (Center of Scientific Computing Ltd., Espoo, Finland) is
acknowledged (M.R.S.).
of the molecule indicate a strong p-electron density delocalization
through the six-membered rings surrounding the Cu ion. As a re-
sult of this, some bond distances and angles of the free ligands
are affected by the Cu coordination (Table 4). For example, the
C–O, C@N and C–N distances are slightly elongated upon the Cu
complex formation. The Oꢂ ꢂ ꢂN distance, on the other hand, is con-
siderably longer (0.17–0.26 Å) in the complexes than in the corre-
sponding free ligands. The longest Oꢂ ꢂ ꢂN distance is found in 2
Appendix A. Supplementary data
(2.824 Å) which might be caused by the p–p stacking of two phe-
nyl groups in the structure. Similar long distance (2.814 Å) has
been found in bis(N-3,5-di-tert-butyl-phenylsalicylaldimina-
to)Cu(II) where the ligands are in a cis-arrangement [3]. In the
complexes the C6–C7–N8 (3.6–6.8°) and O1–C1–C6 (1.6–2.7°) an-
gles are expanded when compared to the corresponding free li-
gands. This is probably related to the lack of the intramolecular
Nꢂ ꢂ ꢂH–O hydrogen bond in the complex structures.
CCDC 826272, 826274, 826275, 826276 and 826277 contain the
supplementary crystallographic data for complexes 3, 6, 7, 7T and
9, respectively. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
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